Vector halftone methods are typically used to render color separations for the printing of images in an inkjet printer. In such a method, a single stochastic screen is used to render all colors. For each pixel, the input color values for the primary colors are used to calculate the primary and secondary colors for a pixel. Then these colors are added in some predetermined order and the sum is compared to a threshold after each addition. When the sum exceeds the threshold for a pixel, the pixel is assigned to that color.
The conversion of primary color inputs to primary, secondary, tertiary, and quaternary color values for a pixel may be done in different ways. In one process, the conversion is done in a manner that minimizes ink by eliminating color under black and limiting secondary color output to only one of the secondary colors, red, green, or blue for a pixel. The letters R, G, and B are used in this document to denote the red, green, and blue secondary colors, respectively. Other process may use the approach described above and add color to be put under black to enhance the optical density of the image. Another method removes only enough color to eliminate any tertiary colors and then the secondary colors are apportioned to make the minimum amount of blue needed to ensure no tertiary colors are assigned and the green secondary color is maximized over red with the remaining amount of needed secondary color.
Vector halftone methods can be affected by ink drop placement errors. Ink drop placement errors occur because inkjets may not eject ink drops perpendicularly with respect to the face of the printhead. These off-angle ink drops land at locations other than were they are expected to land. Some printheads are oriented at an angle with respect to the width of the image receiving member. This angle is sometimes known as printhead roll in the art and printhead roll may also contribute to ink drop placement errors. Consequently, reducing the effect of ink drop placement on the images printed by a printer is a worthwhile goal in inkjet printing systems.